This application claims the benefit of Korean Patent Application No. 2001-25694, filed in Korea on May 11, 2001, which is hereby incorporated by reference as if fully set forth herein.
1. Field of the Invention
The present invention relates to a method of crystallizing amorphous silicon, and more particularly, to a sequential lateral solidification (SLS) crystallization method.
2. Discussion of Related Art
Polycrystalline silicon (p-Si) and amorphous silicon (a-Si) are often used as the active layer material for thin film transistors (TFTs) in liquid crystal display (LCD) devices. Since amorphous silicon (a-Si) can be deposited at a low temperature to form a thin film on a glass substrate, amorphous silicon (a-Si) is commonly used in liquid crystal displays (LCD). Unfortunately, amorphous silicon (a-Si) TFTs have relatively slow display response times that limit their suitability for large area LCD.
In contrast, polycrystalline silicon TFTs provide much faster display response times. Thus, polycrystalline silicon (p-Si) is well suited for use in large LCD devices, such as laptop computers and wall-mounted television sets. Such applications often require TFTs having field effect mobility greater than 30 cm2/Vs together with low leakage current.
A polycrystalline silicon film is composed of crystal grains having grain boundaries. The larger the grains and the more regular the grain boundaries the better the field effect mobility. Thus, a silicon crystallization method that produces large grains, ideally a single crystal, would be useful.
One method of crystallizing amorphous silicon into polycrystalline silicon is sequential lateral solidification (SLS). SLS crystallization uses the fact that silicon grains tend to grow laterally from the interface between liquid and solid silicon. With SLS, amorphous silicon is crystallized using a laser beam having a magnitude and a relative motion that melts amorphous silicon such that the melted silicon forms laterally grown silicon grains upon recrystallization.
FIG. 1A is a schematic configuration of a conventional sequential lateral solidification (SLS) apparatus, while FIG. 1B shows a plan view of a conventional mask 38 that is used in the apparatus of FIG. 1A. In FIG. 1A, the SLS apparatus 32 includes a laser generator 36, a mask 38, a condenser lens 40, and an objective lens 42. The laser generator 36 generates and emits a laser beam 34. The intensity of the laser beam 34 is adjusted by an attenuator (not shown) in the path of the laser beam 34. The laser beam 34 is then condensed by the condenser lens 40 and is then directed onto the mask 38.
The mask 38 includes a plurality of slits xe2x80x9cAxe2x80x9d through which the laser beam 34 passes, and light absorptive areas xe2x80x9cBxe2x80x9d that absorb the laser beam 34. The width of each slit xe2x80x9cAxe2x80x9d effectively defines the grain size of the crystallized silicon produced by a first laser irradiation. Furthermore, the distance between each slit xe2x80x9cAxe2x80x9d defines the size of the lateral grain growth of amorphous silicon crystallized by the SLS method. The objective lens 42 is arranged below the mask and reduces the shape of the laser beam that passed through the mask 38.
Still referring to FIG. 1A, an X-Y stage 46 is arranged adjacent to the objective lens 42. The X-Y stage 46, which is movable in two orthogonal axial directions, includes an x-axial direction drive unit for driving the x-axis stage and a y-axial direction drive unit for driving the y-axis stage. A substrate 44 is placed on the X-Y stage 46 so as to receive light from the objective lens 42. Although not shown in FIG. 1A, it should be understood that an amorphous silicon film is on the substrate 44, thereby defining a sample substrate.
To use the conventional SLS apparatus, the laser generator 36 and the mask 38 are typically fixed in a predetermined position while the X-Y stage 46 moves the amorphous silicon film on the sample substrate 44 in the x-axial and/or y-axial direction. Alternatively, the X-Y stage 46 may be fixed while the mask 38 moves to crystallize the amorphous silicon film on the sample substrate 44.
When performing SLS crystallization, a buffer layer is typically formed on the substrate. Then, the amorphous silicon film is deposited on the buffer layer. Then, the amorphous silicon is crystallized as described above. The amorphous silicon film is usually deposited on the buffer layer using chemical vapor deposition (CVD). Unfortunately, that method produces amorphous silicon with a lot of hydrogen. To reduce the hydrogen content the amorphous silicon film is typically thermal-treated, which causes de-hydrogenation, which results in a smoother crystalline silicon film. If de-hydrogenation is not performed, the surface of the crystalline silicon film is rough, and the electrical characteristics of the crystalline silicon film are degraded.
FIG. 2 is a plan view showing a substrate 44 having a partially-crystallized amorphous silicon film 52. When performing SLS crystallization it is difficult to crystallize all of the amorphous silicon film 52 at once because the laser beam 34 has a limited beam width, and because the mask 38 also has a limited size. Therefore, with large substrates the mask 38 is typically arranged numerous times over the substrate, while crystallization is repeated for the various mask arrangements. In FIG. 2, an area xe2x80x9cCxe2x80x9d that corresponds to one mask position is defined as a block. Crystallization of the amorphous silicon within a block xe2x80x9cCxe2x80x9d is achieved by irradiating the laser beam several times.
Crystallization of the amorphous silicon film will be explained as follows. FIGS. 3A to 3C are plan views showing one block of an amorphous silicon film being crystallized using a conventional SLS method. In the illustrated crystallization it should be understood that the mask 38 (see FIGS. 1A and 1B) has three slits.
The length of the lateral growth of a grain is determined by the energy density of the laser beam, by the temperature of substrate, and by the thickness of amorphous silicon film (as well as other factors). The maximum lateral grain growth should be understood as being dependent on optimized conditions. In the SLS method shown in FIGS. 3A to 3C, the width of the slits is twice as large as the maximum lateral grain growth.
FIG. 3A shows an initial step of crystallizing the amorphous silicon film using a first laser beam irradiation. As described with reference to FIG. 1A, the laser beam 34 passes through the mask 38 and irradiates one block of an amorphous silicon film 52 on the sample substrate 44. The laser beam 34 is divided into three line beams by the three slits xe2x80x9cA.xe2x80x9d The three line beams irradiate and melt regions xe2x80x9cD,xe2x80x9d xe2x80x9cE,xe2x80x9d and xe2x80x9cFxe2x80x9d of the amorphous silicon film 52. The energy density of the line beams should be sufficient to induce complete melting of the amorphous silicon film 52.
Still referring to FIG. 3A, after complete melting the liquid phase silicon begins to crystallize at the interfaces 56a and 56b between the solid phase amorphous silicon and the liquid phase silicon. Namely, lateral grain growth of grains 58a and 58b proceeds from the un-melted regions to the fully-melted regions. Lateral growth stops in accordance with the width of the melted silicon region when: (1) grains grown from interfaces collide near a middle section 50a of the melted silicon region; or (2) polycrystalline silicon particles are formed in the middle section 50a as the melted silicon region solidifies sufficiently to generate solidification nuclei.
Since the width of the slits xe2x80x9cAxe2x80x9d (see FIG. 1B) is twice as large as the maximum lateral growth length of the grains, the width of the melted silicon region xe2x80x9cD,xe2x80x9d xe2x80x9cE,xe2x80x9d or xe2x80x9cFxe2x80x9d is also twice as large as the maximum lateral growth length of the grains. Therefore, lateral grain growth stops when the polycrystalline silicon particles are formed in the middle section 50a. Such polycrystalline silicon particles act as solidification nuclei in a subsequent crystallization step.
As discussed above, the grain boundaries in directionally solidified silicon tend to form perpendicular to the interfaces 56a and 56b between the solid phase amorphous silicon and the liquid phase silicon. As a result of the first laser beam irradiation, crystallized regions xe2x80x9cD,xe2x80x9d xe2x80x9cE,xe2x80x9d and xe2x80x9cFxe2x80x9d are formed in one block. Additionally, solidification nuclei regions 50a are also formed.
As mentioned before, the length of lateral grain growth attained by a single laser irradiation depends on the laser energy density, the temperature of substrate, and the thickness of the amorphous silicon film. In the above-mentioned first laser beam irradiation, the grains generated by the lateral growth typically have a length ranging from 1.5 to 3 micrometers.
FIG. 3B shows crystallizing the amorphous silicon film using a second laser beam irradiation. After the first laser beam irradiation, the X-Y stage or the mask 38 moves in a direction opposite to the lateral grain growth of the grains 58a or 58b (in FIG. 3A) by a distance of several micrometers, which is the same as or less than the maximum length of the lateral grain growth. Then, the second laser beam irradiation is conducted. The regions irradiated by the second laser beam are melted and crystallized as described above. The silicon grains 58a and 58b or/and the nuclei regions 50a generated by the first laser beam irradiation serve as seeds for the second crystallization. Thus, the lateral grain growth proceeds in the second melted regions. Silicon grains 58c formed by the second laser beam irradiation continue to grow adjacent to the silicon grains 58a formed by the first laser beam irradiation, and silicon grains 58d grown from an interface 56c are also formed. The lateral growth of these grains 58c and 58d stops when the nuclei regions 50b are formed in a middle section of the silicon region melted by the second laser beam irradiation.
Accordingly, by repeating the foregoing steps of melting and crystallizing, one block of the amorphous silicon film is crystallized to form grains 58e as shown in FIG. 3C.
The above-mentioned crystallization processes conducted within one block are repeated block by block across the amorphous silicon film 52. Therefore, the large size amorphous silicon film is converted into a crystalline silicon film. While generally successful, the conventional SLS method described above has disadvantages.
Although the conventional SLS method produces large size grains, the X-Y stage or the mask must repeatedly move a distance of several micrometers to induce lateral grain growth. Therefore, the time required to move the X-Y stage or the mask 38 occupies a major part of the total crystallization time. This significantly decreases manufacturing efficiency.
FIG. 4 is a plan view of a mask 60 that is used in another SLS method. The mask 60 has light slits xe2x80x9cGxe2x80x9d and light absorptive areas xe2x80x9cH.xe2x80x9d Although the mask 60 is similar to the mask 38, the width of the lateral stripe-shaped slits xe2x80x9cGxe2x80x9d is less than twice the maximum lateral grain growth length. Due to the smaller width of the slits xe2x80x9cGxe2x80x9d the lateral grain growth stops when the grains generated at the interface between the un-melted regions and the fully-melted regions collide. In contrast to the crystallization described in FIGS. 3A to 3C, solidification nuclei regions 50a and 50b are not formed when using the mask 60.
The SLS using the mask 60 will now be discussed. As described with reference to FIG. 1A, the laser beam 34 passes through the mask 60 and irradiates the amorphous silicon film on the sample substrate 44. The laser beam 34 is divided into three line beams (because there are three slits xe2x80x9cGxe2x80x9d). Those line beams are reduced by the objective lens 42 to create beam patterns on the amorphous silicon film. As crystallization proceeds, the beam patterns move in an X-axis direction. Because of the X-axis directional movement, crystallization is conducted along a length of the beam pattern. As previously described, the X-Y stage 46 or the mask 60 moves by a distance of several millimeters (mm). The larger movement reduces the processing time when compared to the SLS method described with reference to FIGS. 3A to 3C.
FIGS. 5A to 5C are plan views showing an amorphous silicon film being crystallized using the mask shown in FIG. 4. It is assumed that the mask 60 has three slits. As mentioned above, the length of the lateral grain growth is determined by the energy density of the laser beam 34, the temperature of the substrate, the thickness of the amorphous silicon film, etc. Thus, lateral grain growth is maximized under optimized conditions. In FIGS. 5A to 5C, it should be understood that the width of the slits xe2x80x9cGxe2x80x9d (in FIG. 4) is smaller than twice the maximum length of the lateral grain growth.
FIG. 5A shows an initial step of crystallizing the amorphous silicon film. Referring to both FIGS. 1A and 5A, the laser beam 34 emitted from the laser generator 36 passes through the mask 60 (which replaces the mask 38) and irradiates a first block E1 of an amorphous silicon film 62 deposited on the sample substrate 44. The laser beam 34 is divided into three line beams by the slits xe2x80x9cG.xe2x80x9d The three line beams irradiate and melt regions xe2x80x9cI,xe2x80x9d xe2x80x9cJ.xe2x80x9d and xe2x80x9cKxe2x80x9d of the amorphous silicon film 62. Since each of the melted regions xe2x80x9cIxe2x80x9d, xe2x80x9cJxe2x80x9d and xe2x80x9cKxe2x80x9d corresponds to a slit xe2x80x9cG,xe2x80x9d the width of the melted regions xe2x80x9cI,xe2x80x9d xe2x80x9cJ.,xe2x80x9d or xe2x80x9cKxe2x80x9d is less than twice the maximum lateral grain growth. The energy density of the line beams should be sufficient to induce complete melting of the amorphous silicon film.
The liquid phase silicon begins crystallize at the interfaces 66a and 66b between the solid phase amorphous silicon and the liquid phase silicon. Namely, the lateral grain growth of the grains 68a and 68b proceeds from un-melted regions adjacent the fully-melted regions. Then, lateral growth stops in accordance with the width of the melted silicon region when the grains 68a and 68b collide in middle lines 60a of the melted silicon region. The grain boundaries in directionally solidified silicon tend to form perpendicular to the interfaces 66a and 66b between the solid phase amorphous silicon and the liquid phase silicon. As a result of the first laser beam irradiation, the first block E1 is partially crystallized. Thereafter, by way of moving the X-Y stage where the substrate is mounted, the beam patterns moves in the X-axis direction by a distance of several millimeters (mm). Thus, the second irradiation is conducted and the second block E2 is partially crystallized. The crystallization in the X-axis direction is then repeatedly carried out.
As a result of the first to third laser beam irradiations described in FIG. 5A, crystallized regions xe2x80x9cI,xe2x80x9d xe2x80x9cJ,xe2x80x9d and xe2x80x9cKxe2x80x9d are formed in the first to third blocks E1, E2 and E3, each of which corresponds to the mask 60 of FIG. 4, such that crystallized silicon grain regions xe2x80x9cI,xe2x80x9d xe2x80x9cJ.,xe2x80x9d and xe2x80x9cKxe2x80x9d result.
In FIG. 5B, after the first laser beam irradiation, the X-Y stage or the mask moves in a direction opposite to the lateral growth of the grains 68a or 68b of FIG. 5A by a distance of several or several tens of micrometers more or less than the maximum length of the lateral growth. Namely, crystallization is conducted block by block in a Y-axis direction. Therefore, the regions irradiated by the laser beam are melted and then crystallized in the manner described in FIG. 5A. At this time, the silicon grains 68a or/and 68b grown by the first to third laser beam irradiations serve as seeds for this crystallization, and thus the lateral grain growth proceeds in the melted regions in the Y-axis direction. Silicon grains 68c formed by the sequential lateral solidification (SLS) continue to grow adjacent to the silicon grains 68a of FIG. 5A, and silicon grains 68d solidified from an interface 66c are also formed. These grains 68c and 68d collide with each other at a middle line 60b of the silicon regions melted by the Y-axial laser beam irradiation, thereby stopping the lateral grain growth.
Accordingly, by repeating the foregoing steps of melting and crystallizing the amorphous silicon, the blocks E1, E2 and E3 of the amorphous silicon film become crystallized to form grains 68e as shown in FIG. 5C. FIG. 5C is a plan view showing a crystalline silicon film that resulted from lateral growth of grains to predetermined sizes.
The conventional SLS methods described in FIGS. 3A to 3C and 5A and 5C have some disadvantages. The conventional SLS method takes a relatively long time to crystallize the amorphous silicon film, thereby causing the decrease of the manufacturing yield. Furthermore, due to the width of slit of the mask, the length of lateral grain growth is limited.
Accordingly, the present invention is directed to a method of crystallizing an amorphous silicon film using a sequential lateral solidification (SLS) that substantially obviates one or more of problems due to limitations and disadvantages of the related art.
An advantage of the present invention is to provide a sequential lateral solidification (SLS) method that saves time.
Another advantage of the present invention is to provide a method of crystallizing an amorphous silicon layer having increased manufacturing yield.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from that description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the method particularly pointed out in the written description and claims hereof as well as the appended drawings.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a method for crystallizing an amorphous silicon film in a sequential lateral solidification (SLS) apparatus includes the steps of locating a substrate in a sequential lateral solidification (SLS) apparatus, wherein the substrate has an amorphous silicon film. Then, irradiating the amorphous silicon film using a laser beam that passes through a mask, wherein the mask is installed in the SLS apparatus and includes a light absorptive portion and a plurality of light-transmitting portions each shaped like tiers, wherein each light-transmitting portion has a plurality of rectangular sub-portions that are adjacent to each other, wherein a width of each rectangular sub-portion is several micrometers, for example, greater than 1 micrometer and less than 10 micrometers, and wherein a length of each rectangular sub-portion ranges from several hundreds micrometers (e.g., 200 micrometers) to sever millimeters. Then, forming first crystallized regions that correspond to the light-transmitting portions, wherein each crystallized region has a plurality of silicon sub-portions, each having a first grain region, a second grain region, and a middle section. The grains of the first and second grain regions grow vertically against the interface between liquid and solid silicon. Then, moving the mask in a transverse direction for a next crystallization, with the movement being the length of the rectangular sub-portions. Then, performing a second crystallization to form second crystallized regions wherein the second grain regions of the first crystallized regions continue their growth. The, repeating forming the first crystallized regions, moving the mask and performing the second crystallization until the amorphous silicon film is crystallized in the transverse direction. The method for crystallizing an amorphous silicon film further includes a step of moving the mask in a longitudinal direction after finishing the transverse directional crystallization, and then further includes a step of conducting a second transverse directional crystallization after moving the mask in the longitudinal direction.
In the above method, the adjacent rectangular sub-portions of the mask constitute a step. The step between adjacent rectangular sub-portions is larger than or smaller than the maximum length of lateral growth of the grains. The width of each rectangular sub-portion is equal to or less then twice the maximum length of the lateral grain growth. At this time, the middle section of the each silicon sub-portion is a collision region where grains laterally growing from opposite side interfaces collide. Alternatively, the width of each rectangular sub-portion can be equal to or larger then twice the maximum length of lateral grain growth. The middle section of the each silicon sub-portion then acts as a nucleus region.
In another aspect, a mask for crystallizing an amorphous silicon film using a sequential lateral solidification (SLS) apparatus includes a light absorptive portion that blocks a laser beam, and a plurality of light-transmitting portions through which the laser beam passes, each of which is tier shaped. Each light-transmitting portion includes a plurality of adjacent rectangular sub-portions. A width of each rectangular sub-portion is equal to or several micrometers less then twice the maximum lateral grain growth. Alternatively, the width of each rectangular sub-portion can be equal to or larger then twice the maximum lateral grain growth. Adjacent rectangular sub-portions of the mask constitute a step. The step between the adjacent rectangular sub-portions is larger than or smaller than the maximum lateral grain growth. A length of each rectangular sub-portion ranges from several hundreds micrometers to several millimeters.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.